1. Field of the Invention
The present invention relates to balancing of the input voltage in a polyphase rectifying apparatus having a circuit structure in which single-phase rectifiers are star-connected.
2. Description of the Related Art
FIGS. 6 and 7 are block diagrams showing and explaining the structure of a conventional three-phase three-wire rectifying apparatus. In FIG. 7, reference numerals 21, 22, and 23 indicate single-phase rectifiers, and reference numerals 24, 25, and 26 indicate converters. According to such star-connected single-phase rectifiers, the input voltage of each single-phase rectifier is given by xe2x80x9cline voltage/{square root over ( )}3xe2x80x9d. Therefore, if the input source is AC 400V, the input voltage of each single-phase rectifier is approximately 230V, and accordingly, a 200V system can be used in the design of the single-phase rectifiers.
However, for such simple star-connected single-phase rectifiers, the input voltage is not balanced and the merit of reducing the input voltage to a 1/{square root over ( )}"" level is not efficiently obtained. The reason for this will be explained below.
FIG. 8 is a diagram showing the general structure of a stabilized power supply which outputs a stabilized (i.e., fixed) output voltage. That is, the stabilized power supply is a xe2x80x9cblack boxxe2x80x9d for changing the impedance Zi (observed from the input) according to the output voltage Vo and internally converting and outputting the internal energy which depends on the impedance.
That is, when the output voltage decreases to below a specific level, the stabilized power supply decreases the impedance Zi observed from the input so as to transmit a larger amount of energy, thereby increasing the output voltage Vo. Conversely, when the output voltage increases above a specific level, the stabilized power supply increases the impedance Zi so as to decrease the transmitted energy, thereby decreasing the output voltage Vo.
FIG. 6 shows a generalized structure of the single-phase rectifiers which function as a stabilized power supply. That is, the single-phase rectifier is also regarded as a black box for changing the impedance Zin (observed from the input) according to its own output voltage. For convenience of explanation, it is assumed that the impedance Zic of the converter observed from the input is not changed. This is an ideal condition. In addition, each single-phase rectifier has the same characteristics, that is, it is assumed that the three functions xe2x80x9cZiu=f(Vou)xe2x80x9d, xe2x80x9cZiv=f(Vov)xe2x80x9d, and xe2x80x9cZiw=f(Vow)xe2x80x9d, are the same. This is also an ideal condition.
Under these ideal conditions, if the output voltages Vou, Vov, and Vow of the single-phase rectifiers are the same, the impedances Ziu, Ziv, and Ziw observed from the input are the same. It is obvious that when the impedances Ziu, Ziv, and Ziw are all the same, the input voltages Viu, Viv, and Viw are also the same.
When such an ideal balanced state is slightly unbalanced, for example, when the output voltage Vou of the single-phase rectifier 11 slightly decreases, the impedance Ziu observed from the input is slightly decreased so as to stabilize the output voltage, as explained above. Accordingly, the input voltage Viu is also slightly decreased because the input voltages Viu, Viv, and Viw depend on the impedance ratio.
Here, the energy transmitted through the single-phase rectifier 11 is (Viu)2/Ziu. Therefore, when the input voltage Viu decreases, the transmitted energy also decreases, and when the transmitted energy decreases, the output voltage Vou also decreases. Accordingly, the impedance Ziu decreases again, thereby decreasing Viu, Vou, . . . , that is, such a decreasing operation (i.e., positive feedback) is repeated and the input voltage Viu finally decreases to a lower limit.
Therefore, the circuit shown in FIG. 6 is essentially unstable, and if the output voltage is slightly changed due to a disturbance, this change is amplified. Therefore, even if the characteristics of the single-phase rectifiers and converters are equalized, it is impossible to actually maintain a balanced state of the input voltages.
As explained above, the problem of unbalanced input voltages is not solved only by simply providing single-phase rectifiers for each phase.
FIG. 9 shows a conventional circuit which employs a step-up chopper type active filter for each single-phase rectifier in FIG. 6, and FIGS. 10A to 10C are diagrams showing the simulated characteristics of the circuit. In FIG. 9, resistors 127, 128, and 129 have been substituted for the converters on the assumption that the impedance observed from the input is the same for each phase.
When the resistors 127, 128, and 129 have the same resistance, the input voltages of the single-phase rectifiers are balanced as explained above. Here, it is assumed that the resistance of the resistor 127 is higher by 1% than the resistances of the other resistors so as to provide an imbalance between the converters.
FIGS. 10A to 10C show the results of a simulation for the case that the resistor 127 has a resistance 1% higher than the other resistors. FIG. 10A shows the waveform of the input voltage of each single-phase rectifier, that is, the waveforms which are measured by voltmeters 76, 77, and 78. As clearly shown in FIG. 10A, the input voltages of the single-phase rectifiers are unbalanced. The input voltage of the phase corresponding to the resistor 128 is almost 0, and accordingly, the input voltages of the other phases are higher than the specific (i.e., suitable) level.
FIG. 10B shows the waveform of the output voltage of each single-phase rectifier, that is, the waveforms which are measured by voltmeters 130, 131, and 132. The output voltage of the phase corresponding to the resistor 128 is lower, while the output voltages of the other two phases are higher. Here, the specific output voltage is 400V, and the output voltages of the other two phases are considerably higher than 400V.
FIG. 10C shows the waveform of the line current of each phase, that is, the waveforms which are measured by ammeters 73, 74, and 75. The waveforms indicate that a function as an active filter is not active but a function close to condenser input is active.
Japanese Unexamined Patent Application, First Publication No. Hei 6-217551 discloses a technique for solving imbalances in the input voltage, in which a controller 30 (see FIG. 5 of the publication) is provided. The input voltage of each phase is balanced by controlling the load of each single-phase rectifier by using the controller 30.
In addition, PCT International Publication No. WO94/27357 discloses another conventional technique for solving imbalances in the input voltage, in which a virtual neutral point N is generated using resistive elements, and a control circuit for making the voltage between the connection point of the single-phase rectifiers and the virtual neutral point zero is provided. That is, the electric potentials of the connection point of the single-phase rectifiers and the virtual neutral point are the same, and which results in a balanced state.
PCT International Publication No. WO99/57800 also discloses another conventional technique for solving imbalances in the input voltage, in which means A2 (see FIG. 3 of the publication) for generating an artificial neutral point is provided, and the connection point NA2 between the single-phase rectifiers is the artificial neutral point. Such means for generating an artificial neutral point is limited to a magnetic component which makes the sum of the vectors of the magnetic fluxes inside the system zero.
In consideration of the above circumstances, an object of the present invention is to provide a polyphase rectifying apparatus which substantially solves the instability of the single-phase rectifiers without adding a special circuit element (as added in the conventional systems), thereby balancing the input voltages of the single-phase rectifiers.
Therefore, the present invention provides a polyphase rectifying apparatus used in a two or more-phase system, comprising:
a single-phase rectifier assigned to each phase, and having first and second input terminals, wherein the first input terminal of each single-phase rectifier is connected to the corresponding phase and the second input terminals of each of the single-phase rectifiers are connected to each other; and
a converter assigned to each phase, wherein the output from the single-phase rectifier of the relevant phase is input into the corresponding converter, wherein:
the single-phase rectifiers for all the phases are controlled by a common control signal.
In the above structure, the second input terminal of each single-phase rectifier may be connected to a neutral point.
Typically, each single-phase rectifier functions as a step-up chopper type active filter.
Preferably, the converters of all the phases are controlled by a common control signal.
Typically, the common control signal is the output voltage from one of the single-phase rectifiers. The common control signal may be the maximum or minimum voltage among the output voltages from the single-phase rectifiers. In addition, the common control signal may be the average of the output voltages from the single-phase rectifiers.
Typically, the impedances of the single-phase rectifiers observed from the input are controlled by the common control signal.
When the converters are also controlled by a common control signal, typically, the impedances of the converters observed from the input are controlled by the common control signal.
According to the present invention, the impedances of the star-connected single-phase rectifiers observed from the input can be balanced. Therefore, if no neutral point is provided, the input voltages of the single-phase rectifiers are balanced, while if a neutral point is provided, the neutral-line currents can be reduced.
In the balanced state of the input voltages of the single-phase rectifiers, the input voltage of each single-phase rectifier is given by xe2x80x9cline voltage/{square root over ( )}3xe2x80x9d. Therefore, if the input source is AC 400V, the input voltage of each single-phase rectifier is approximately 230V, and accordingly, a 200V system can be used in the design of the single-phase rectifiers.